In industries, primarily metallurgical industries, in which molten metal is handled there is a great need for systems for measuring or estimating the position or average level of the interface between the molten metal and non-conducting media in a metallurgical vessel such as a ladle, tundish, mould, a furnace, etc. Due to the specific harsh conditions encountered in connection with such industry, such as high temperatures, corrosive materials, different electrically conducting materials in the vessels and surroundings, up to now it has proved to be difficult to provide a general technique for such level measurement.
Some dominating techniques for level estimation in the metallurgical industry include weight measurements, radioactive systems and electromagnetic systems. Weight measurements are indirect, and radioactive systems have a limited range and do not separate between conductive or non-conductive media. Electromagnetic systems have been successfully deployed in a variety of vessels, such as ladles, tundishes and furnaces. These vessels have a metal housing, often provided with an inner ceramic lining to manage heat and abrasive materials. Electromagnetic sensors may be placed behind the lining so as to be protected from the excessive heat from the molten metal. The electromagnetic sensors include a combination of one or more transmitter coils and one or more receiver coils. The transmitter coil may be driven at a low frequency of 100 Hz to a few kHz to generate a time-varying magnetic field. Many lining materials are transparent at these frequencies which allows the magnetic field to reach the molten metal and induce eddy currents therein. The eddy currents generate fields that induce an electromotive force (emf) in the receiving coil, which may be detected to represent the amount of molten metal within the extent of the coils. The transmitter and receiver coils are traditionally designed as flat square coils arranged on the sides of the vessel or on top of each other on one side of the vessel. This results in installation limitations.
The magnetic field generated by a coil is inherently non-linear, where the strength of the magnetic field drops with distance (R) to the coil conductor by 1/R near the coil conductor and by 1/R3 when the distance R is large relative to the extent of the coil. Unless special care is taken, this results in a non-linear transfer function, i.e. a non-linear dependence between electromotive force in the receiver coil and vertical filling level in the vessel. Such non-linearity is e.g. illustrated in U.S. Pat. No. 4,144,756, in which the transmitter and receiver coils are arranged coaxially and separated in the axial direction in the lining of a metallurgical vessel. The dependence of measurement signal on filling level is highly non-linear with inflexion points being formed at the horizontal axes of the coils.
Several techniques have been applied to make the transfer function more linear relative to the vertical filling level.
In U.S. Pat. No. 4,475,083, a flat single-turn transmitter coil and a flat single-turn receiver coil are arranged vertically in the lining of a furnace wall, so as to overlap each other and extend parallel to the periphery of the molten metal. A detection circuit is connected to the transmitter coil to detect a phase shift between the alternating current supplied to the transmitter coil and the resulting electromagnetic alternating field at the receiver coil. This coil arrangement is stated to result in a measurement signal which may sufficiently linear to signify unambiguity between signal value and measured value.
In U.S. Pat. No. 4,708,191, a rectangular transmitter coil is installed in the lining of a metallurgical vessel to extend in the horizontal and vertical directions of the vessel. At least two rectangular receiver coils are aligned with and staggered vertically within the transmitter coil to cover various surface areas of the transmitter coil. The coil arrangement may be designed to generate a measurement signal proportional to level, with crossover points generated in the measurement signal by the placement of the receiving coils within the transmitter coil.
The coil arrangements of the prior art have proven to be less suited in many practical situations, e.g. when measuring on a constantly changing three-dimensional interface between a conducting and a primarily non-conducting medium, e.g. a tumultuous or turbulent top surface of molten metal. This is partly due to the fact that the vertical range with linear signal dependence is substantially less than the physical height of the coil arrangement. When turbulent portions of the top surface reach outside this vertical range, the induced eddy currents in these turbulent portions may drive the measurement signal in opposite direction relative to the actual physical movement. This may lead to significant errors in the measured filling level. Mother problem is that the extent of the vertical range may drift over time, making it difficult to maintain the techniques for linearization of the measurement signal.
The prior art also includes U.S. Pat. No. 4,887,798, EP0187993 and EP0111228, which disclose techniques for detecting a flow of molten metal through an outlet in a metallurgical vessel. Transmitter and receiver coils are arranged concentrically around the opening, and the receiver coil is operated to detect electromotive forces originating from eddy currents generated in the molten metal by an alternating current through the transmitter coil. This enables detection of presence or absence of molten metal at the level of the concentric coils.
The prior art also comprises EP0186584, which discloses level detection in a horizontally arranged cylindrical metal pipe. A pair of conductors are wound onto the outer surface of the metal pipe to form a concentrically arranged pair of transmitting and receiving coils. The amount of conductive material inside the pipe is measured based the electromotive forces generated in the receiving coil by an alternating current through the transmitter coil.